This invention relates to an oxygen generating device and to a system incorporating an oxygen generating device.
In order for passengers or aircrew in an aircraft to breath when exposed to ambient atmospheric pressure at elevated altitudes, it is necessary to provide a supply of breathing gas enriched with oxygen.
One means of achieving this is to carry within the airframe a supply of compressed oxygen gas, but particularly in a small aircraft, where space is at a premium, and/or in an aircraft where the added weight of the gas bottle containing the compressed oxygen gas is significant, this is not acceptable.
To reduce weight and space requirements another means is to carry within the airframe liquid oxygen. Liquid oxygen systems give rise to space and weight penalties and also a requirement for liquid oxygen to be available for replenishment of the liquid oxygen supply at a ground station.
More recently oxygen-enriched gas has been produced on-board of the aircraft by a so-called on-board oxygen generating system (OBOGS) based on pressure swing technology using a zeolite molecular sieve material to separate oxygen from air. This requires at least two zeolite beds which have to be sequentially cycled through on-stream/generating and off-stream/purge cycles. A limitation of such systems is that theoretically the maximum oxygen concentration obtainable in the product gas is 95% unless additional means are provided for the removal of argon and other trace gases from the supply air which is normally bleed air from a compressor stage of an engine powering the aircraft.
Increasing attention is now being given to ceramic membrane technology in provision of a system which will generate substantially 100% oxygen product gas or highly oxygen-enriched product gas of breathable quality for use in aerospace and other breathing applications. Such gas will hereinafter be referred to as being "oxygen rich", and the residual gas, will be referred to as being "oxygen depleted".
Certain ceramic materials, which are so-called ionic conductors of oxygen, become electrically conductive at elevated temperatures due to the mobility of oxygen ions within the crystal lattice. Since these materials are only conductive to oxygen ions, an external electric circuit providing electronic conduction is needed. Temperatures in the order of at least 600 K are required to obtain sufficient ionic conductivity.
Such ceramic oxygen generating devices may comprise one or more ceramic membranes through which an electrical current is passed, whilst ambient air is supplied to one face of the membrane which allows oxygen in the supply air to diffuse through the membrane by ionic transport when the membrane is at the required elevated temperature, and be recovered on the other side of the membrane.
To ensure that the ambient air entering the device does not cool the membrane and prevent it from efficiently being heated to a minimum operating temperature, the ambient supply air is pre-heated typically by passing the supply air through a heat exchanger to which hot oxygen rich gas and/or hot oxygen depleted gas delivered from the oxygen generating device is fed, so that the cooler ambient air is heated.
The electrical current passing through the material has a heating effect on the material and on the air passing through the device, and at least after an initial warm up period, the oxygen generating device is self sustaining at a temperature above the minimum operating temperature at which air is separated into its oxygen rich and oxygen depleted gas components.
The amount of ambient air fed to the oxygen generating device, (and electrical current passed through the material) can be controlled as necessary, to ensure that the demand for the oxygen rich gas supply is met, but in previous arrangements it has been necessary to provide complex active control means to control the electrical current and hence the temperature of the device, so that overheating of the oxygen generating device is avoided.
With at least some commonly used ceramic membrane materials, there is a higher electrical resistance through the material at lower temperatures, and so the magnitude of electrical current which can pass through the material to cause a heating effect, is dependent on the temperature of the material, which in turn depends at least in part on the temperature of the gas mixture delivered to the material. The inventors have realised that this property of the material can be utilised to avoid overheating of the oxygen generating device without an active control means being required to control the electrical current.